(1) Field of the Invention
The present invention relates to: a solid-state imaging device which includes two-dimensionally arranged light-receiving elements, vertical transfer units, and a horizontal transfer unit, and outputs image signals; a driving method of the device; and a camera using the device, and particularly to a solid-state imaging device having a still picture imaging mode and a moving picture imaging mode; a driving method of the device; and a camera using the device.
(2) Description of the Related Art
Charge-Coupled Device (CCD) imaging sensors have been known as solid-state imaging devices each of which has a plurality of light-receiving elements for converting incident light into electrical charges and outputs the charges as image signals. Furthermore, digital still cameras using such solid-state imaging devices have become popular. In recent years, technologies of high pixel density in solid-state imaging devices have been developed, so that some digital still cameras can provide images whose resolution is higher than resolution of silver halide photographs.
The conventional solid-state imaging device includes a plurality of photoelectric conversion units, a plurality of vertical transfer units, a horizontal transfer unit, and an output unit. The photoelectric conversion units have color filters arranged in Bayer pattern array. Each of the vertical transfer units is formed corresponding to each column of the photoelectric conversion units, and vertically transfers signal charges read from each of the photoelectric conversion units to the horizontal transfer unit. The horizontal transfer unit horizontally transfers the signal charges received from the vertical transfer units. The output unit amplifies and then outputs the signal charges provided from the horizontal transfer units.
Most of digital still cameras have functions of imaging moving pictures as well as still pictures. The number of pixels in one still picture is generally more than 4 million pixels, for example, but for imaging moving picture, the number of pixels are usually reduced (hereinafter, referred to also as “thinned”) to achieve necessary frequency (more than 30 frames per second, for example). A typical example of such pixel thinning in a vertical direction is selecting one photoelectric conversion unit from three neighboring photoelectric conversion units and reading signal charges from the selected photoelectric conversion unit to the vertical transfer unit.
Another example of the vertical pixel thinning is a method disclosed in Japanese Patent Application Publication No. 9-18792 (hereinafter, referred to as Patent Reference 1). By this method, to the horizontal transfer unit, signal charges are continuously and simultaneously transferred from neighboring vertical transfer stages in vertical transfer stages including in a vertical transfer unit. Thereby, signal charges in the neighboring vertical transfer stages are mixed in the horizontal transfer unit, so that it is possible to reduce the number of pixels in a vertical direction and thereby increase frame frequency more.
Moreover, Japanese Patent Application Publication No. 2004-180284 (hereinafter, referred to as Patent Reference 2) discloses a solid-state imaging device in which pixels are able to be reduced (thinned) in a horizontal direction. This solid-state imaging device has a vertical final stage in each vertical transfer unit. The vertical final stages in every (2n+1) columns (in every three columns, for example) have the same structure of transfer electrodes. Each vertical final stage in the (2n+1) columns has at least two transfer electrodes which are independent from electrodes of other columns, so that each column can be independently controlled to transfer signal charges from the vertical final stage to the horizontal transfer unit. For example, in the case where pixels of two different colors are alternately arranged in a row as in Bayer pattern array, every other pixels of the same color in a horizontal direction in the (2n+1) columns are selected to read signal charges, and the signal charges are transferred from the vertical final stages to the horizontal transfer unit and mixed together in the horizontal transfer unit. By repeating the above processing (2n+1) times, it is possible to reduce the number of pixels in a horizontal direction to one-(2n+1)th.
Thus, in the case where moving pictures are imaged by a solid-state imaging device whose total pixels are numerous, the pixels are reduced without lowering frame frequency. In this case, for preventing image quality defects, it is desirable to keep a balance between horizontal resolution and vertical resolution by reducing the number of pixels in both horizontal and vertical directions.
However, it is impossible to combine the structure disclosed in Patent Reference 1 for vertical pixel reduction with the structure disclosed in Patent Reference 2 for horizontal pixel reduction. More specifically, in the structure disclosed in Patent Reference 2, every other pixels of the same color in a horizontal direction in the vertical final stage are selected, and signal charges are read and transferred from the selected pixels and then mixed in the horizontal transfer unit. However, during the above processing, it is impossible at the same time to continuously transfer signal charges from all a plurality of vertical transfer units to the horizontal transfer unit, as the structure disclosed in Patent Reference 1. Therefore, if pixels are to be reduced also in a vertical direction when pixels are reduced in a horizontal direction using the technology of Patent Reference 2, the vertical pixel reduction has been realized by performing empty transfer using empty vertical transfer stages (empty transfer states) which are formed in some of the vertical transfer units and from which no signal charges are read out from photoelectric conversion units.